This invention relates generally to sea chests and more particularly to covers and liners used with sea chests.
In many water-going vessels, particularly ocean-going vessels, the hull is provided with superstructure compartments that receive and expel water. These compartments are known generally as “sea chests” and usually take form of cavities of various shapes. Sea chests are sealed with respect to the interior of the vessel and are frequently an integral part of the hull. One example of a function of a sea chest would be to house valves designed to regulate the flow of water into and out of the vessel for use as ballast and/or engine cooling. Another example is housing maneuvering thrusters that cause water to be forced from the compartment. Yet another example would be housing sensors used to determine the speed of the vessel or the depth of the water below the vessel. If such valves, thrusters, or sensors, which must be in contact with the water in order to operate correctly, were placed directly on the exterior of the vessel's hull, they would be subject to catastrophic stresses from the water flowing by, from debris, or from any substantive structure that the hull might contact. Accordingly, it is common to place such water-contacting equipment within a sea chest, which can be filled with water but provides a compartment that is essentially protected by the vessel's hull.
To prevent debris or floating material of any kind from entering a sea chest, a cover shaped to the contour of the ship's hull at the location of the sea chest, is mounted over the chest opening. These covers generally comprise a grate or a plurality of holes that permit water to enter or exit the sea chest, dependent upon the pressure differential between the sea chest interior and the hull exterior. The cover thus acts as a filter to keep debris out of the sea chest while allowing free passage of water therethrough.
Sea chest covers are frequently made of cast iron or steel to provide sufficient structural strength. When the equipment within a sea chest must be repaired or otherwise worked on, the sea chest cover must first be removed. This is either done in dry dock or as part of an underwater operation. However, because sea chest covers are typically heavy, awkward pieces of metal, removal and replacement of such covers can be a very difficult task—particularly when carried out underwater.
Another major difficulty with traditional sea chests is “bio-fouling,” that is, the growth of algae, barnacles, or other marine organisms on the interior of the sea chest as well as the cover. At a microscopic level, the relatively rough metal surfaces make for excellent attachment sites for such organisms. Cleaning the cover and the interior of the sea chest is a difficult task. Furthermore, such fouling diminishes flow of water into and out of the sea chest, and can even clog the sea chest and jeopardize the onboard operations that utilize the water. In cold waters, ice crystals can also grow on sea chest covers and clog the sea chests. Heating techniques to remove ice build up tend to be prohibitively expensive.
Another difficulty arises from the fact that metal surfaces of water-going vessels are susceptible to corrosion. Corrosion is particularly of concern with ocean-going vessels because the high concentration of salt in seawater accelerates corrosion. This is especially the case with sea chest covers formed from cast iron, as the covers suffer from deposits of salts and associated pitting due to electrolytic reactions stemming from the dissimilar metals of the hull and the covers in a bath of essentially dissociated ionic salts. Generally, corrosion is combated by protecting all water-contacting surfaces—including all surfaces of the sea chest and the cover—with some form of sealant. For the most part, that sealant is paint. Under the conditions typically experienced, vessel hulls must be painted frequently because any breaks in the thin protective coat of paint become a starting point for corrosion. This remedy is at best moderately successful and is very expensive because the procedure requires the vessel to be in dry dock.
Another problem associated with metal components in water is that of erosion caused by the flow of water and foreign particles contained in the water. The erosion caused by scouring effect of a vessel's movement in the water flakes off paint and pits the metal surfaces of the sea chest covers. Over time, the cover may be eroded enough to require replacement. Furthermore, pitting from erosion and corrosion enhances the growth of marine organisms by supplying a foothold or anchor to the metal surfaces.
Yet another concern related to the use of metal sea chest covers is noise and vibration. When a vessel moves through the water, turbulent flow rates impart vibrational energy to the vessel's hull, including the metal sea chest covers. Such vibrations will generate noise and can cause operational difficulties. Water flowing through the cover will also generate noise. These noises all contribute to the acoustic signature of the vessel. The lack of energy-absorbing capability is a particular problem where it is desirable to avoid the transmittal of sound waves to the surrounding water.